Has anyone had experience of using both a heater and a heat pump? I am looking to extend the pool season and use gas for Dec and March/April (in Washington DC) - I already have a heat pump to keep costs down during the rest of the year.
Let me see if I understand. You have a heatpump and want to spend money to buy a gas heater - is that correct?
I cannot understand why you would want to do this as it is much more costly to use gas to heat the pool. I have a heatpump, and recently added some solar panels so as to reduce electric consumption further.
There would be no problem doing what you are proposing - I just don't understand why you would want to.
The reason for the gas is to heat the pool when the ambient temperature is below 40 as the heat pump will not operate well below that temperature. I do have a pool cover and am looking to try to use the pool for a few more months.
I don't see any reason that wouldn't work. You will probably need to manually tell the heaters which one should be running at any given point, unless you want to spring for a fairly fancy control system. Just keep in mind that the cost of heating a pool with gas when the temperature is low enough to cause problems for a heat pump is going to be dramaticaly high.
This is done quite frequently and usually where a heat pump just can't keep up, or shuts down due to cold temps. Actually the outer evaporator freezes up from the condensate water and quits. Most heat pump manufacturers offer a reverse defrost cycle that will thaw this frost, to allow operation down into colder weather conditions. AquaCal's models are called the Ice Breaker.
Fortunately this dual heater set up is done mostly for diehard swimmers in cold climate regions and is only for brief periods of the year so the cost is not tooooo bad.
You'll just need to install the gas heater after the heat pump and at a lower thermostat setting than the heat pump. When temps fall below the heat pump's ability to maintain it, the gas heater will kick on.